Lecture 2 6.263/1637 The Data Link Layer: Framing and Error Detection Eytan Modiano MIT LIDS
Lecture 2 6.263/16.37 The Data Link Layer: Framing and Error Detection Eytan Modiano MIT, LIDS Eytan Modiano Slide 1
Data Link Layer (DLC) Responsible for reliable transmission of packets over a link Framing: Determine the start and end of packets(sec 2.5) Error Detection: Determine when a packet contains errors ( sec 2.3) Error recovery: Retransmission of packets containing errors(sec 2. 4) DLC layer recovery May be done at higher layer
Data Link Layer (DLC) • Responsible for reliable transmission of packets over a link – Framing: Determine t he star t and end of packets (sec 2.5) – Error Detection: Determine w hen a packet contains errors (sec 2.3) – Error recovery: Retransmissio n of packets containing errors (sec 2..4) DLC lay er recovery May be done at higher lay er Eytan Modiano Slide 2
Framing 010100111010100100101010100111000100 Where is the data?? Three approaches to find frame and idle fill boundaries 1)Character oriented framing 2)Length counts fixed length 3)Bit oriented protocols( flags)
Framing _____________________________________ 010100111010100100101010100111000100 Where is the DATA?? • Three approaches to find frame and idle fill boundaries: 1) Character oriented framing 2) Length counts - fixed length 3) Bit oriented protocols (flags) Eytan Modiano Slide 3
Character Based Framing Frame SYNSYN STX Header Packet ETX CRC SYN SYN SYN is synchronous idle STX is start text ETX is end text Standard character codes such as ascll and ebcDic contain special communication characters that cannot appear in data Entire transmission is based on a character code
Character Based Framing Frame SYN SYN STX Header Packet ETX CRC SYN SYN SYN is synchronous idle STX i s start text ETX is end text • Standard character codes such as ASCII and EBCDIC contain special communication characters that cannot appear in data • Entire transmission is based on a character code Eytan Modiano Slide 4
Issues with Character Based Framing Character code dependent How do you send binary data? Frames must be integer number of characters Errors in control characters are messy NOTE: Primary Framing method from 1960 to-1975
Issues With Character Based Framing • Character code dependent – How do you send binary data? • Frames must be integer number of characters • Errors in control characters are messy NOTE: Primary Framing method from 1960 to ~1975 Eytan Modiano Slide 5